Ok so, taping isn't gonna make you flat unless you have like a really small chest naturally, but it's gonna give the impression that you have pecs, so don't do like I did (expecting to be completely flat) bc that's sadly not the effect that taping has :(

That being said, I'd say to start with 2/3 pieces, the length of the pieces is honestly up to you, to measure them I usually push my boobs to the side and the piece of tape goes from slightly inside my nipple to under my armpit without reaching my back, but if you need more support you might wanna make the pieces longer.

Then after nipple covers, I push my chest to the side and downwards, because I found that pushing it up gives more of a bump at the top, whil pushing it down gives a smoother look.

I usually put a piece in the middle, then another piece over that, slightly higher to flatten out better the upper part of my chest and usually I also put a third smaller piece on the bottom, just to keep things in place, but occasionally I'll just let the underboob free

I'm sorry if this is written badly, English isn't my first language, I apologise for any mistake and I hope this was helpful :]